Received July 2, 2008; Revision received September 8, 2008
Angiostatins, kringle-containing fragments of plasminogen, are potent
inhibitors of angiogenesis. Effects of three angiostatin forms, K1-3,
K1-4, and K1-4.5 (0-2 µM), on the rate of native
Glu-plasminogen activation by its physiological activators in the
absence or presence of soluble fibrin were investigated in
vitro. Angiostatins did not affect the intrinsic amidolytic
activities of plasmin and plasminogen activators of tissue type (tPA)
and urokinase type (single-chain scuPA and two-chain tcuPA), but
inhibited conversion of plasminogen to plasmin in a dose-dependent
manner. All three angiostatins suppressed Glu-plasminogen activation by
tcuPA independently of the presence of fibrin, and the inhibitory
effect increased in the order: K1-3 < K1-4 < K1-4.5. The
inhibitory effects of angiostatins on the scuPA activator activity were
lower and further decreased in the presence of fibrin. Angiostatin K1-3
(up to 2 µM) had no effect, while 2 µM angiostatins
K1-4 and K1-4.5 inhibited the fibrin-stimulated Glu-plasminogen
activation by tPA by 50 and 100%, respectively. The difference in
effects of the three angiostatins on the Glu-plasminogen activation by
scuPA, tcuPA, and tPA in the absence or presence of fibrin is due to
the differences in angiostatin structures, mechanisms of action, and
fibrin-specificity of plasminogen activators, as well as due to the
influence of fibrin on the Glu-plasminogen conformation. Angiostatins
in vivo, which mimic plasminogen-binding activity, can inhibit
plasminogen activation stimulated by various proteins (including
fibrin) of extracellular matrix, thereby blocking cell migration and
angiogenesis. The data of this work indicate that the inhibition of
Glu-plasminogen activation under the action of physiological
plasminogen activators by angiostatins can be implicated in the complex
mechanism of their antiangiogenic and antitumor action.
KEY WORDS: plasminogen, plasminogen activator, plasmin,
angiostatin, inhibition of plasminogen activation